Access True Area 53:
To unlock 'true area 53' (the hidden zone), beat the game with all the emeralds for every character.You'll warp into space as Super Sonic for the final fight against Robotnik

Play as Cream:
Beat the Leaf Forest level as Sonic.

Play as Tails:
Beat the Music Plant level as Sonic.

Play as Knuckles:
Beat the Sky Canyon level as Sonic.

Play as Amy:
Beat the game with all seven Chaos Emeralds as Cream, Knuckles, Sonic, and Tails.

Thanks to Revolution reader jonathan mcgregor!